Response of isolated muscle spindles to single transient stretches.

Abstract

ABSTRACTThe smallest stretch of isolated frog muscle spindles which regularly evoked an afferent discharge was 8 ± 2 μ measured as the displacement of a graphite granule placed above the equatorial region 5–8 mm from the distal tendop of the muscle. The threshold deformation (T) was independent of an increase in the rate of deformation from 0.016 xT/msec to 1.0 × T/msec and of an increase in the initial elongation from 10 to 25 per cent above equilibrium length. The rate below which the spindle did not respond to stretch was 0.01 × T/msec. With low rates of stretch (0.016–0.03 × T/msec) the afferent response occurred before the deformation was completed and with higher rates (0.1–1.0 × T/msec) after it was completed.A second afferent response was elicited when the deformation exceeded 2 times threshold (rate of stretch: 0.1–0.3 × T/msec) or 3–4 times threshold (rate of stretch: 0.6–2.7 × T/msec). The shortest interval between two afferent responses was 2 msec.Depending on the timing of the spontaneous afferent activity and the spontaneous intrafusal contractions the afferent response evoked by a stretch was abolished or its latency was shortened or increased.